The hbd mutation in double gene dose in mice is associated with hypochromic microcytic anemia. The phenotype has been partially characterized previously and by recent work in this laboratory. The anemia does not respond to iron or pyridoxine therapy, and is probably not a form of thalassemia. The gene has been securely established in our mouse colony and further work is proposed. It is planned to (i) fully characterize the phenotype; and (ii) to elucidate the basic pathogenetic mechanism through hematologic, biochemical and biosynthetic investigations. It is aimed thereby, (iii) to establish the existence and nature of a genetically controlled step in iron metabolism not hitherto known. This finding is likely to be of general biological significance. The hbd mutant may have a human disease analogue and will certainly provide insight into human iron metabolism and its disorders. Finally, (iv) the gene locus will be mapped by classic breeding methods, contributing further to the gene map of the mouse.